The present invention relates to rotary electrical machines such as electrical generators and electric motors and to particularly advantageous components for use in such machines.
UK Patent Specification GB-A-2 222 031 describes an axial field electrical generator capable of operation at very high speeds. However, certain features of this known design have been perceived to have some drawbacks. In response to these, our UK patent specification No. GB-A-2 261 327 describes and claims certain improvements in rotary electrical machines. Both these known designs employ rotors having substantially equi-angularly spaced magnets retained by a retention ring and stators provided with respective electrical windings.
The generator described in GB-A-2 222 031 employs air cooling for the stators. The air is directed by means of radial channels entering at the rim. The channels conduct air towards the centre of the stators and back again to the rim. As a result, on the return path the air is already hot and so performs little cooling. Hence this arrangement means that the components of the rotors, especially the magnets and the temperature-critical retention ring are subjected to air which has already been heated due to passage through the stators.
On the other hand, GB-A-2 261 327 discloses a stator comprising at least one radial channel for ducting of cooling air. The channel has an entrance at or substantially near the rim of the stator and an exit at or substantially near the centre of the stator.
The latter cooling arrangement has now been improved, in accordance with a first aspect of the present invention, by provision of a rotary electrical machine comprising at least one stator and at least one adjacent rotor, a space being provided between said stator and said rotor, the stator being provided with at least one set of at least two substantially parallel radial channels for ducting of cooling air, said set of radial channels extending between a first position at or substantially near the rim of a winding region of the stator and a second position at or substantially near the centre of the winding region, the electrical machine further comprising cooling means for causing cooling air to pass through said set of radial channels and to pass through said space between said stator and said rotor.
The cooling system of the first aspect of the present invention has the advantage that more surface area for cooling inside the stator. This is due to the electrical windings described below with reference to the third aspect of the present invention of reduction in windage losses due to reduction in pressure in the stator-rotor gap.
The cooling means may be arranged to cause the cooling air to enter the set of radial channels via the first position and exit via the second position. In this case, the cooling air preferably passes through the space between the stator and the rotor after exiting the set of radial channels.
Alternatively, the cooling means may be arranged to cause the cooling air to enter the set of radial channels via the second position and exit via the first position. In this case, the cooling air preferably passes through the space between the stator and the rotor before entering the set of radial channels.
This means that the rotor components, especially the ring and magnets are cooled first with the highest temperatures occurring only in the stationary stator coils which can withstand higher temperature due to the absence of mechanical stresses.
The cooling means may be arranged to provide pressurised cooling air to the at least one set of radial channels at one position to blow the cooling air through the set of radial channels to the other position.
Alternatively, the cooling means could be arranged to subject the set of radial channels at one position to a reduced pressure to suck the cooling air through the set of radial channels from the other position.
The machine may comprise a drive shaft, and the cooling means may then comprise an evacuation compressor actuable by means of the drive shaft. The cooling means may, on the other hand, comprise a pump, for example, a remote pump. The cooling means may alternatively comprise a fan.
The stator is preferably provided with electrical windings and the at least one rotor preferably has a plurality of substantially equi-angularly spaced magnets.
The set of radial channels can follow a substantially straight path or a curved or meandering path between the first and second positions, although the general direction of the set of channels will be radial.
In accordance with a second aspect of the present invention, there is provided a rotary electrical machine comprising at least one stator, the stator being provided with at least one radial channel for ducting of cooling air, said radial channel extending between a first position at or substantially near the rim of a winding region of the stator and a second position at or substantially near the centre of the winding region, the electrical machine further comprising cooling means for causing cooling air to enter the radial channel via the second position and exit via the first position.
The cooling means may be arranged to provide pressurised cooling air to the at least one radial channel at the second position to blow the cooling air through the radial channel to the first position. Alternatively, the cooling means may be arranged to subject the first position of the at least one radial channel to a reduced pressure to suck the cooling air through the radial channel from the second position.
The cooling means may comprise a pump or a fan. Alternatively, the machine may comprise a drive shaft and the cooling means may comprise an evacuation compressor actuable by the drive shaft.
The machine is preferably provided with a respective space between the at least one stator and at least one adjacent rotor, the cooling means being arranged to cause cooling air to pass through the space between the stator and the rotor. The cooling air preferably passes through the space between the stator and the rotor before entering the radial channel.
The radial channel may follow a substantially straight path between the first and second positions. Alternatively, the channel may follow a meandering path between the first and second positions, although the general direction of the channel will be radial.
The at least one stator preferably comprises electrical windings, and the at least one adjacent rotor preferably comprises a plurality of substantially equi-angularly spaced magnets.
The stator is preferably provided with at least one set of radial channels, each set comprising at least two substantially parallel radial channels. In fact, the stator may comprise three layers of electrical windings, at least one radial channel of a set of radial channels being disposed between adjacent windings.
The stator described in GB-A-2 222 031 has windings which are standard wave windings as conventionally used in electrical machines. That is to say, the respective windings for each phase are in separate overlapping planes.
To assist miniaturisation, the arrangement described in GB-A-2 261 327 has a stator comprising a plurality of windings for various phases of electric current, the windings being substantially in the same plane.
The machine described hereinbelow as embodiments of the first and second aspects of the present invention contains a stator which has a particularly advantageous arrangement of electrical windings. Thus, a third second aspect of the present invention provides a stator for an axial field electrical machine, the stator comprising electrical windings arranged as coil sectors disposed substantially equi-angularly in a general circular pattern, wherein at least some of the coil sectors are wound in a generally spiral fashion when viewed in the direction of axis of symmetry of the said generally circular pattern.
A stator constructed in the form of the third aspect of the present invention facilitates simpler manufacture, a greater surface area being provided for cooling, and a greater output than the wave-winding arrangements described in GB-A-2 222 031 and GB-A-2 262 327. Moreover, it allows more flexibility in the choice of magnet shape.
Preferably, each of the coil sectors is wound in a generally spiral fashion. The term xe2x80x9cgenerally spiral fashionxe2x80x9d includes the configuration of a pinched spiral, although a substantially regular spiral is also possible, as well as other minor distortions of the general spiral shape. In the case of a pinched spiral, the spiral shape may be pinched inwardly at its part closest to the centre of the generally circular pattern. Such a pinched part may have a flattened portion substantially facing the centre of the generally circular pattern.
The electrical windings may be provided with connections for receipt of, or output of, a three-phase electrical current. However, two-phase or other arrangements are also possible.
For the rotor, the machine described in GB-A-2 222 031 utilises a carbon fibre-reinforced hoop for retaining magnets in an angularly spaced arrangement. However, the rotor described in GB-A-2 261 327 has a retaining hoop formed from reinforced carbon fibres. The fibres extend around the hoop at a non-normal angle relative to the axis of symmetry of the hoop.
The preferred embodiments of an electrical machine described hereinbelow also discloses a particularly advantageous rotor. This rotor comprises; in accordance with a fourth aspect of the present invention, a plurality of magnets substantially equi-angularly spaced around an axis of rotation, at least some of said magnets being generally circular when viewed in the direction of the axis of rotation and having a cut-away portion.
The magnets for the rotor according to the fourth aspect of the present invention may, for example, be made by grinding the periphery of a conventional button magnet. In any event, the fourth aspect of the present invention reduces manufacturing costs when compared to cutting a special shape for small to medium production volumes, e.g. a special shape as disclosed in GB-A-2 222 031 or GB-A-2 261 327.
Preferably, each of the magnets is generally circular when viewed in the direction of axis of rotation and has a cut-away portion. Most preferably, the edge of each magnet which defines the cut-away portion is convexly curved.
As described hereinbelow with respect to the fourth aspect, the convexly curved edge of each magnet faces outwardly relative to the axis or rotation. The convexly curved edge has substantially the same degree of curvature as the periphery of the rotor. The magnets are retained by a retention hoop.